class Spring {
  float cx, cy;        // rest position
  float x, y;          // current position
  float vx=0, vy=0;    // velocity in the two directions   
  float r = 10;        // radius
  float k = 0.05;      // spring constant
  float d = 0.75;      // damping
  float g = 1.0;       // gravity
  
  // Initialize a Spring at rest position (x0,y0)
  Spring(float x0, float y0)
  {
    x = x0; y = y0;
    cx = x0; cy = y0;
  }

  void draw()
  {
    // A "mass" at the current position, and a line to the center
    noStroke();
    ellipse(x,y,r*2,r*2);
    stroke(128);
    line(cx,cy, x,y);
  }

  // Update the spring, with a new center
  void update(float ncx, float ncy)
  {
    cx = ncx; cy = ncy;
    
    // Spring equations for both x and y; gravity for y
    vx -= k * (x-cx);
    vx *= d;
    x += vx;
    vy -= k * (y-cy);
    vy += g;
    vy *= d;
    y += vy;
  }
}